1. Field of the Invention
The present invention relates to an image forming apparatus which includes a single image carrier and a plurality of developers, and forms a color image.
2. Description of the Related Art
A color image forming apparatus, which includes a single photosensitive member (image carrier) and developers corresponding to the number of colors of developing agents, adopts, for example, a color image forming method (to be referred to as a rotary method hereinafter) which repetitively executes image forming cycles as many as the number of colors of the developing agents.
In the rotary method, since developed images of respective colors are required to be overlaid on each other on a belt- or drum-shaped transfer member used to perform primary transfer of images, one or more marks serving as reference positions are arranged on the transfer member. Therefore, the rotary method is able to overlay developed images on the transfer member by starting image formations with reference to a detection signal of these marks by a sensor (to be referred to as a transfer belt reference signal hereinafter).
In the control of the conventional rotary method, when the image forming apparatus receives a print request from, for example, a PC, preparation operations for, for example, activating a scanner motor, transfer belt, and photosensitive member are performed. Furthermore, in the preparation operations, print image data is rasterized to raster image data to allow to form electrostatic latent images by means of light beams. After completion of the preparation operations, upon reception of the transfer belt reference signal, an output enable signal of the raster image data is output after an elapse of a predetermined time period, and image data is output from an image controller in synchronism with a BD signal. After that, formation of a latent image, development, and transfer processing are repetitively executed as many as the number of developing colors according to image data, thereby forming a color image as the developing agent images of the respective developing colors are overlaid on each other on the transfer belt. Such control of the rotary method is disclosed in, for example, Japanese Patent Laid-Open No. 09-290534.
In recent color printers, higher throughputs are demanded. As described above, in the image forming apparatus of the conventional rotary method, the transfer belt reference signals have to be received so as to overlay toner images of respective colors to have a precise positional relationship. However, when the marks pass through the sensor immediately before completion of initialization operations, they are detected at a delayed timing, thus delaying the start of print processing.